Europa's ocean holds the potential for finding life beyond earth. Better yet, if life does exist on Europa, it will be the second planet within one solar system, greatly increasing the odds for life throughout the galaxy.
Europa's potential was first realized in 1979 when Voyager 2 flew by. Scientists were surprised to see that the surface of Europa wasn't scarred by the usual maculation of impact craters. This suggested that the surface of Europa was relatively young, which most likely meant geologic activity.
The explanation was simple: as Europa orbits Jupiter, its larger companion pulls and stretches it which generates heat in the interior and causes tectonic activity. These tidal forces could generate enough heat to prevent the freezing of water below the surface, creating a layer of liquid ocean that separates the icy crust from the rock mantle.
|Image credit: JPL/NASA|
The next journey to Europa revealed more, nearly (but not quite) confirming the existence of a liquid ocean, see the magnetic field section for more. Some have estimated from this second mission that the ocean is vast, containing more than twice the volume of water on Earth.
More recent studies have also suggested the existence of wator vapor plumes and geothermal vents beneath the surface, further increasing the probability of life.
Of course, a liquid ocean is not the only possibility for the observed phenomana, and other possibilities still exist.